The present invention generally relates to a dual state mirror assembly for use with automobiles, trucks, buses and other types of vehicles, primarily as a rearview mirror. The dual state mirror of the present invention is particularly useful as an exterior mirror for larger trucks, including those in classes 6, 7 and 8, as well as buses, and will be described in this general context for convenience of illustration. However, the general principles described will also be useful in conjunction with other types of mirrors, for various types of vehicles, and for other applications such as laser technology.
Early attempts to reduce the glare associated with rearview mirrors for vehicles included the use of prismatic features, and electronic features, including electrochromic and dichroic liquid crystal display technologies.
In the case of prismatic technology, the rearview mirror is provided with two independent states of reflectance using a prismatic element including a high reflectance surface for daytime use, and a low reflectance surface for nighttime use. To operate such a mirror, the driver manually adjusts the position of the prismatic element, depending upon the time of day.
In both positions the driver actually views both prism surfaces, with different images. In the daytime position, the high reflectance surface yields sufficient light to make the dimmer image on the low reflectance surface unnoticeable. In the nighttime position, the high reflectance surface is pointed to a dark region in the vehicle (typically the roof) so as not to interfere with the dimmer image on the low reflectance surface.
In the case of electrochromic and dichroic liquid crystal display technologies, the dimming function is achieved electronically, and is therefore automated.
All of these methods, however, have some key drawbacks. Mirrors that incorporate prismatic surfaces suffer from unpredictable optical quality. If the lighting conditions inside the vehicle become less than optimum (i.e., vehicles travelling closely behind and illuminating the interior), the rearview mirror can produce multiple images. More importantly, prismatic mirrors can not be used for exterior applications since there is no satisfactory dark region to which the high reflectance surface can be directed.
Electrochromic mirrors also tend to produce multiple images. In addition, such mirrors have slow response times and tend to alter the color of the reflected images. Although electrochromic mirrors can be used as exterior mirrors, they are very costly for the relatively large areas associated with exterior mirror applications (particularly for larger vehicles such as trucks and buses).
The use of dichroic mirrors is essentially precluded by regulation. In 1992, the U.S. National Highway Traffic Safety Administration (NHTSA) issued a regulation that all multiple reflectance mirrors revert to a high reflectance state in the event of electrical failure. This condition could not be effectively met with dichroic mirrors.
Earlier efforts to develop a useful dual state rearview mirror, prior to electrochromic and dichroic LCD technologies, employed paired, plate glass elements to achieve the desired result. Examples of these devices may be found with reference to U.S. Pat. Nos. 3,574,446 (Moore); 3,836,235 (Russell); 4,371,235 (Locke, Sr.); and 4,560,259 (Russell). Each of these mirror systems includes an enclosure with a plate glass face, and a mirrored element positioned within the enclosure and spaced from the plate glass face. The mirrored element is capable of movement (i.e., rotation) between daytime and nighttime positions which are primarily determined by the position of the mirrored element relative to the plate glass face of the enclosure.
However, in each case, the disclosed devices are provided with substrate surfaces (plate glass and mirror) which are spaced apart. In addition, the disclosed devices are provided with plate glass and mirrored elements that do not allow for adjustment of the reflective and transmissive properties of the overall system. As a consequence, none of these devices are particularly satisfactory in actual operation. Double images are not uncommon, and the difference between the reflected images from the different mirror surfaces (i.e., elements in parallel, in a daytime mode position) is so great that nighttime use (while in a high reflectance, daytime configuration) is seriously compromised.